RCU Kconfig help text

RCU Kconfig help text

[Valdis.Kletnieks]

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init/Kconfig says:

"choice
        prompt "RCU Implementation"
        default TREE_RCU

config TREE_RCU
        bool "Tree-based hierarchical RCU"
        help
          This option selects the RCU implementation that is
          designed for very large SMP system with hundreds or
          thousands of CPUs.  It also scales down nicely to
          smaller systems.

config TREE_PREEMPT_RCU
        bool "Preemptable tree-based hierarchical RCU"
        depends on PREEMPT
        help
          This option selects the RCU implementation that is
          designed for very large SMP systems with hundreds or
          thousands of CPUs, but for which real-time response
          is also required.

endchoice"

This leaves somebody who has a laptop wondering which choice is best for
a system with only one or two cores that has CONFIG_PREEMPT defined. One
choice says it scales down nicely, the other explicitly has a 'depends on
PREEMPT' attached to it...

(Yes, I realize in practice, the RCU sections on a laptop are probably usually
so short they don't matter in practice.  I finally concluded TREE_PREEMPT was
apparently a rename of CONFIG_PREEMPT_RCU and went with that since that was
working for me before...)


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Re: RCU Kconfig help text

[Paul E. McKenney]

On Sat, Sep 05, 2009 at 09:27:10AM -0400, Valdis.Kletnieks@vt.edu wrote:
> init/Kconfig says:
> 
> "choice
>         prompt "RCU Implementation"
>         default TREE_RCU
> 
> config TREE_RCU
>         bool "Tree-based hierarchical RCU"
>         help
>           This option selects the RCU implementation that is
>           designed for very large SMP system with hundreds or
>           thousands of CPUs.  It also scales down nicely to
>           smaller systems.
> 
> config TREE_PREEMPT_RCU
>         bool "Preemptable tree-based hierarchical RCU"
>         depends on PREEMPT
>         help
>           This option selects the RCU implementation that is
>           designed for very large SMP systems with hundreds or
>           thousands of CPUs, but for which real-time response
>           is also required.
> 
> endchoice"
> 
> This leaves somebody who has a laptop wondering which choice is best for
> a system with only one or two cores that has CONFIG_PREEMPT defined. One
> choice says it scales down nicely, the other explicitly has a 'depends on
> PREEMPT' attached to it...
> 
> (Yes, I realize in practice, the RCU sections on a laptop are probably usually
> so short they don't matter in practice.  I finally concluded TREE_PREEMPT was
> apparently a rename of CONFIG_PREEMPT_RCU and went with that since that was
> working for me before...)

Good point -- I will add the "It also scales down nicely to smaller
systems" to TREE_PREEMPT_RCU.

For -really- small systems, TINY_RCU will hopefully be there at some
point, but it can only handle single-CPU systems.

							Thanx, Paul

Re: RCU Kconfig help text

[Pavel Machek]

On Sat 2009-09-05 13:01:46, Paul E. McKenney wrote:
> On Sat, Sep 05, 2009 at 09:27:10AM -0400, Valdis.Kletnieks@vt.edu wrote:
> > init/Kconfig says:
> > 
> > "choice
> >         prompt "RCU Implementation"
> >         default TREE_RCU
> > 
> > config TREE_RCU
> >         bool "Tree-based hierarchical RCU"
> >         help
> >           This option selects the RCU implementation that is
> >           designed for very large SMP system with hundreds or
> >           thousands of CPUs.  It also scales down nicely to
> >           smaller systems.
> > 
> > config TREE_PREEMPT_RCU
> >         bool "Preemptable tree-based hierarchical RCU"
> >         depends on PREEMPT
> >         help
> >           This option selects the RCU implementation that is
> >           designed for very large SMP systems with hundreds or
> >           thousands of CPUs, but for which real-time response
> >           is also required.
> > 
> > endchoice"
> > 
> > This leaves somebody who has a laptop wondering which choice is best for
> > a system with only one or two cores that has CONFIG_PREEMPT defined. One
> > choice says it scales down nicely, the other explicitly has a 'depends on
> > PREEMPT' attached to it...
> > 
> > (Yes, I realize in practice, the RCU sections on a laptop are probably usually
> > so short they don't matter in practice.  I finally concluded TREE_PREEMPT was
> > apparently a rename of CONFIG_PREEMPT_RCU and went with that since that was
> > working for me before...)
> 
> Good point -- I will add the "It also scales down nicely to smaller
> systems" to TREE_PREEMPT_RCU.
> 
> For -really- small systems, TINY_RCU will hopefully be there at some
> point, but it can only handle single-CPU systems.

If so, call it 'UP_RCU' or 'UNIPROCESSOR_RCU'?
								Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

Re: RCU Kconfig help text

[Paul E. McKenney]

On Sun, Sep 06, 2009 at 08:24:41AM +0200, Pavel Machek wrote:
> On Sat 2009-09-05 13:01:46, Paul E. McKenney wrote:
> > On Sat, Sep 05, 2009 at 09:27:10AM -0400, Valdis.Kletnieks@vt.edu wrote:
> > > init/Kconfig says:
> > > 
> > > "choice
> > >         prompt "RCU Implementation"
> > >         default TREE_RCU
> > > 
> > > config TREE_RCU
> > >         bool "Tree-based hierarchical RCU"
> > >         help
> > >           This option selects the RCU implementation that is
> > >           designed for very large SMP system with hundreds or
> > >           thousands of CPUs.  It also scales down nicely to
> > >           smaller systems.
> > > 
> > > config TREE_PREEMPT_RCU
> > >         bool "Preemptable tree-based hierarchical RCU"
> > >         depends on PREEMPT
> > >         help
> > >           This option selects the RCU implementation that is
> > >           designed for very large SMP systems with hundreds or
> > >           thousands of CPUs, but for which real-time response
> > >           is also required.
> > > 
> > > endchoice"
> > > 
> > > This leaves somebody who has a laptop wondering which choice is best for
> > > a system with only one or two cores that has CONFIG_PREEMPT defined. One
> > > choice says it scales down nicely, the other explicitly has a 'depends on
> > > PREEMPT' attached to it...
> > > 
> > > (Yes, I realize in practice, the RCU sections on a laptop are probably usually
> > > so short they don't matter in practice.  I finally concluded TREE_PREEMPT was
> > > apparently a rename of CONFIG_PREEMPT_RCU and went with that since that was
> > > working for me before...)
> > 
> > Good point -- I will add the "It also scales down nicely to smaller
> > systems" to TREE_PREEMPT_RCU.
> > 
> > For -really- small systems, TINY_RCU will hopefully be there at some
> > point, but it can only handle single-CPU systems.
> 
> If so, call it 'UP_RCU' or 'UNIPROCESSOR_RCU'?

"UP_RCU".  Cute!!!  ;-)

My hope is to drive the RCU selection directly from CONFIG_SMP and
CONFIG_PREEMPT, so that the choice of RCU would be implicit (aside from
things like tracing and fanout).

Seem reasonable, or would you expect anyone to want to hand-select
the RCU implementation?

							Thanx, Paul

Re: RCU Kconfig help text

[Pavel Machek]

> > > > This leaves somebody who has a laptop wondering which choice is best for
> > > > a system with only one or two cores that has CONFIG_PREEMPT defined. One
> > > > choice says it scales down nicely, the other explicitly has a 'depends on
> > > > PREEMPT' attached to it...
> > > > 
> > > > (Yes, I realize in practice, the RCU sections on a laptop are probably usually
> > > > so short they don't matter in practice.  I finally concluded TREE_PREEMPT was
> > > > apparently a rename of CONFIG_PREEMPT_RCU and went with that since that was
> > > > working for me before...)
> > > 
> > > Good point -- I will add the "It also scales down nicely to smaller
> > > systems" to TREE_PREEMPT_RCU.
> > > 
> > > For -really- small systems, TINY_RCU will hopefully be there at some
> > > point, but it can only handle single-CPU systems.
> > 
> > If so, call it 'UP_RCU' or 'UNIPROCESSOR_RCU'?
> 
> "UP_RCU".  Cute!!!  ;-)
> 
> My hope is to drive the RCU selection directly from CONFIG_SMP and
> CONFIG_PREEMPT, so that the choice of RCU would be implicit (aside from
> things like tracing and fanout).
> 
> Seem reasonable, or would you expect anyone to want to hand-select
> the RCU implementation?

Seems reasonable.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html